Github user Tagar commented on the issue:
https://github.com/apache/spark/pull/16228
@wzhfy, it's easier to check validity of these type of expressions when you
look at extreme cases.
Your formula for full outer join cardinality,
> cardinality = MAX(card(A) + card(B), innerCard(AB))
in one of extreme cases when set(A) and set(B) are the same sets, then
calculated cardinality would be 2 times more of the actual cardinality.
While
> full_outer_join_cardinality(table_A, table_B) = cardinality(A) +
cardinality(B) - inner_join_cardinality(table_A, table_B))
will produce correct result.
ps. I find this visualization
http://www.radacad.com/wp-content/uploads/2015/07/joins.jpg very helpful.
https://en.wikipedia.org/wiki/Inclusion%E2%80%93exclusion_principle A U B =
A + B - A \ B
Hope this helps. Thanks!
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